Nice idea, but I suspect it's impossible to complete on a 4x5 grid.
An interesting addition might be the ability to wipe a line out if it's all of the same number.
edit: gah. this is really annoying me now because the controls to slide squares left or right only works about 50% of the time so I end up with lots of the same numbered squares adjacent to each other and no way of merging them. Which makes chaining a few squares together virtually impossible because at least 1 of every 3 left-right slides will fail: